In general, a relay may often be used to cover electromagnetic wave shadow areas or provide an economical extension of coverage to areas which require expensive base stations with the intention of improving quality of service (QoS) of mobile telecommunications. A relay may generally be classified into a wired relay having one side connected to a base station by a wire such as an optical cable and a wireless relay connected to a base station wirelessly.
A relay may include two antennas to perform simultaneous signal transmission and receipt. For an up-link and a down-link, an antenna may include a signal receiving part, which is often called an “acceptor,” and a signal transmitting part, which is often called a “donor.” Among other things, a frequency hopping relay and a transparent relay, both of which allocate different frequencies to their respective donors and acceptors, are not so efficient in terms of frequency, although they have no self-interference. On the contrary, a non-transparent relay behaving like a base station can use the same frequency to transmit/receive signals since signals from other base stations do not interfere with the relay, thereby allowing its donor and acceptor resources to be allocated with the same frequency/time band. In addition, an interference cancellation system relay may allocate its donor and acceptor resources to the same frequency/time band. However, for such an interference cancellation system relay, although it may increase its radio frequency use efficiency over the frequency hopping relay or the transparent relay, when it transmits/receives signals at the same time and with the same frequency, an interference signal, which may occur due to a feedback of a signal transmitted by its donor, may have an effect on the reception performance of its acceptor. Such an interference signal, which may occur when the relay uses the same transmission/reception frequency band, is called a “self-interference (SI),” whose intensity is varied between about −100 dB and about −70 dB depending on ambient geographical features.